The invention relates to the field of microwave bandstop filters.
It finds general application in the processing of microwave signals and more particularly in the microwave receiving stages for which it may be necessary to filter unstable noise signals of generally unknown frequency to be able to analyze and identify useful signals with a good signal-to-noise ratio.
Tunable microwave bandstop filter devices are already known.
For example, in French patent No. 88 03187 published under the number 2 628 571, the THOMSON company proposed a tunable microwave bandstop filter device which comprises:
a main microwave transmission line using stripline technology, and PA1 at least one filter cell comprising a coupled microwave line segment coupled to the main transmission line and arranged parallel to and remotely from the latter, and a tunable LC resonant circuit. PA1 simple use of components and trouble-free micro-electronics integration; PA1 elimination of the microwave plated-through hole; PA1 improvement in performance, particularly with regard to the width of the usable band and the tuning frequency; PA1 wider choice of transmission line technology, in particular the possibility of using suspended substrate technology. PA1 simple use of the resistance and trouble-free and easier micro-electronics integration; PA1 better maintenance of the impedance of the control circuit of the varactor diode at high values at wideband microwave frequencies, and thus improvement in the flatness of the electrical response of the said varactor diode control circuit; PA1 absence of filtering effect in the control circuit of the varactor diode, and therefore possible extension of the use of the filter device into the low frequencies; and PA1 possibility of connecting the control circuit of the varactor diode to the varactor diode itself or to the connection between the tunable LC resonant circuit and the main transmission line.
A signal applied to one of the ends of the transmission line is filtered and the centre frequency of the filtering device thus obtained can be varied by controlling the voltage applied to the tunable LC resonant circuit.
In patent No. 88 03187, the segment of the stripline has an open-circuit first end and a second end connected to ground potential via the tunable LC resonant circuit containing in particular a varactor diode.
Such an arrangement has the following drawbacks.
Firstly, the connection from the second end of the stripline segment to the ground potential via the resonant circuit is made by means of a plated-through hole which, at microwave frequencies, gives rise to residual inductance of a typical value of 0.2 to 0.3 nanohenrys. Now the consequence of this is to limit the application of the filter device to the high-frequency range (typically above 12 GHz). Furthermore, the losses associated with this plated-through hole reduce the overall performance of the filter and in particular the Q-factor of the resonant circuit.
Next, to ensure proper filter operation the access from the plated-through hole to the earth plane must be easy and rapid. But this excludes so-called suspended substrate technology and stripline type technology has to be used exclusively.
Finally, the filter cell supplying the varactor diode in the above-mentioned patent is very difficult to produce for operations in a very wide usable band and/or with a very high tuning speed.